1) Field of the Invention
The invention relates to a method for adjusting a chemical dosage of a pulp processing stage, the method comprising determining the value of a variable in dependence on the chemical dosage prior to adding the chemical dosage, measuring the chemical dosage to be added, determining the value of the variable after adding the chemical dosage, determining, on the basis of the above-mentioned stages, the change in the variable caused by the chemical dosage, determining a model describing the change in the variable as a function of the chemical dosage.
The invention further relates to an apparatus for adjusting a chemical dosage of a pulp processing stage, the apparatus comprising means for determining the value of a variable in dependence on the chemical dosage prior to and after adding the chemical dosage, means for measuring the chemical dosage, calculation means for determining a model describing the change in the variable as a function of the chemical dosage.
The invention relates to adjusting a pulp processing stage, wherein pulp refers to cellulose containing pulp used as a raw material in the manufacture of paper, rayon or the like. The invention particularly relates to adjusting a chemical dosage of a processing stage in a pulp bleaching process.
2) Description of Related Art
As is well known, the purpose of pulp bleaching is to enhance the brightness and cleanliness of pulp by removing colored substances therefrom or by changing such constituents so as to achieve the desired end brightness. Pulp is bleached to a certain level of brightness which depends on the use of the pulp. An important aim of the bleaching process is to keep the brightness level of the bleached pulp as even as possible.
The most significant colored substance of chemically manufactured pulp, i.e. chemical pulp, is residual lignin, which is to be removed from the pulp as completely as possible, in other words pulp is bleached in a manner almost exclusively removing lignin.
The color of pulp can also be bleached in a so-called lignin-saving bleaching process; typically, this method is used in the manufacture of mechanical pulps.
It goes without saying that a bleaching process has to be implemented in an as economical and environmental-friendly manner as possible. The bleaching process is responsible for most of the water consumed and the waste water produced by a sulphate cellulose mill, which makes the role of the bleaching process even more important as regards the environmental load caused by the entire mill. Therefore, the environmental load caused by the chemicals used in bleaching is to be reduced. Such chemicals are also responsible for incurring considerable raw material costs. It is thus extremely important to administer chemical dosages appropriately in order to be able to produce pulp in an environmental-friendly and economical manner.
Conventionally, a chemical dosage of a pulp bleaching process is adjusted by means of a compensated brightness expression feedforward control based on brightness and residual measurements. The method enables suitable dosage adjustment to be achieved in connection with a conventional run, i.e. when chlorine gas is used in the bleaching process. However, the behavior of pulp will eventually undergo changes, and the compensated feedforward does not necessarily respond to these changes. The change in the behavior of the pulp may be short-term, due to e.g. a pulp cooking or washing stage, or it can be more permanent, due to e.g. the characteristics of the raw wood material. A chemical dosage based on the conventional feed-forward is thus not necessarily optimal for a particular situation, which means that the cost-effectiveness and environmental friendliness of the process are reduced and the quality of the pulp becomes uneven.
A method is known from U.S. Pat. No. 6,153,050 for optimizing a chemical dosage of the first stage of a bleaching process. The method is based on a fixed model modeling the reaction cinetics of the chemical used, measuring the characteristics of pulp prior to and after the bleaching stage. On the basis of the model and the measurements, the necessary chemical dosage is determined; if desired, the dosage can be modified by an operator. Since the model is a fixed one, being thus based on the reaction cinetics of a chemical, it does not always describe a real situation in a correct manner because all factors affecting the situation cannot be taken into account by the model.
The method disclosed in CA Patent 999 950 for adjusting a bleaching process applies a model derived from laboratory tests and based on the reaction cinetics of a chemical, the parameters of the model being determined by simulation. This model does not always describe a real situation correctly, either because a real situation under pulp mill conditions also comprises several other factors than those that can be taken into account in the model.
The method disclosed in WO 98/28488 for controlling a bleaching process uses neural networks for modeling the bleaching stage. The modeling examines continuous electromagnetic spectra measured from pulp that are used for providing characteristic quantities for the pulp. On the basis of both the characteristic quantities and laboratory measurements, a process model describing the behavior of the pulp is determined. The method is complex, an essential part of determining the model consisting of time-consuming laboratory measurements that do not necessarily describe the characteristics of the pulp being adjusted under pulp mill conditions.
An article titled xe2x80x9cAdvanced sequence kappa factor control, Part I: DE kappa controlxe2x80x9dby Perala and Kirby, TAPPI Journal, Vol. 84, No. 4, p. 67, discloses a method utilizing model predictive control (MPC) in adjusting a chemical dosage of the first two stages in a bleaching plant. The method comprises providing a predictive process model and continuously measuring the kappa number of pulp. This method does not enable correct control, either since the fixed model of the method is incapable of taking all failure situations of the process into account.
An object of the present invention is to provide a novel and improved method and apparatus for adjusting a chemical dosage of a pulp processing stage.
The method of the invention is characterized by
determining a performance index for the model,
comparing the performance index of the model with the performance indices of certain previously determined models,
bringing into use the model that produced the best performance index in the comparison, and
determining the necessary chemical dosage by means of the model put to use.
Furthermore, the apparatus of the invention is characterized in that the apparatus further comprises
means for determining a performance index for the model,
means for comparing the performance index of the model with the performance indices of certain previously determined models, and
means for bringing into use the model that produced the best performance index in order to determine the chemical dosage.
The idea underlying the invention is that a performance index is determined for a model, the performance index of the model is compared with the performance indices of certain previously determined models, the model that produced the best performance index in the comparison is brought into use, and the necessary chemical dosage is determined by means of the model put to use. The idea underlying an embodiment of the invention is that the performance of a new model is tested with respect to the models in a model bank by arranging the models in measurement data about the change in a variable caused by a chemical dosage, the measurement information being located in a certain history window, and that the new model is introduced into the model bank if the error produced by the model is smaller than an error produced by a previously determined model taken into account in the comparison, and the model that produced the largest error is removed from the model bank. Furthermore, the idea underlying another embodiment of the invention is that the model is linear in terms of parameters, and that the model is determined using linear regression. Furthermore, the idea underlying still another embodiment of the invention is that the number of model banks is two or more, and that in connection with a current run, a model bank specific to the current run is put to use.
An advantage of the invention is that the adjustment of a chemical dosage becomes more accurate since the model is updated according to the special characteristics of the pulp just being run. When the kind of raw material is changed, the specific model banks enable a model appropriate for a new run to be selected immediately. A further advantage of the invention is that it is simple to implement, and that it can be applied to different pulp processing stages in a versatile manner. A further advantage is that only the decisive aspect, i.e. the bleaching of pulp, is measured.